This invention relates to a process for producing purified liquid crystals, a liquid crystal composition, and a liquid crystal cell and a liquid crystal display device using these purified liquid crystals and the liquid crystal composition.
In order to improve display properties of liquid crystal display devices, it is important to improve the resistivity of liquid crystals and to improve the voltage retention rate of liquid crystal cells. But liquid crystal cells and liquid crystal display devices now used have a problem in that ionic impurities are adsorbed on orientation films made from a polyimide and deteriorate display properties. In order to remove such ionic impurities, various methods are proposed using liquid crystal purifying material. For example, Japanese Patent Unexamined Publication (JP-A) No. 64-87685 discloses a process for removing impurities using an adsorbent such as silica gel, alumina, porous polymer beads made from styrene-divinylbenzene copolymer, ion exchange resins, etc. But since the adsorption ability is so strong, the adsorbent adsorbs not only ionic impurities which are adsorbed on polyimide orientation film and deteriorate liquid crystal display properties, but also other additives such as liquid crystal mixtures having polar groups, chiral agents, etc., resulting in changing the liquid crystal composition and deteriorating the display properties.
On the other hand, when the resistivity of liquid crystals and the voltage retention rate were lowered by contamination of liquid crystals at the time of insertion into a liquid crystal cell or a liquid crystal display device, it was difficult to recover these properties. In order to solve such a problem, JP-A 4-258925 discloses a device for inserting liquid crystals having a liquid crystal reservoir, inner wall of which is covered with a polyimide resin in order to remove impurities by adsorbing on the polyimide resin. But since the contact area with liquid crystals is small, the removal of impurities is insufficient, resulting in failing to obtain long-period display stability, particularly under high temperature circumstances (40.degree.-80.degree. C.). Further, even if the polyimide resin was ground to a powder, the specific surface area was still insufficient to remove the impurities.